Clutches are disengageable torque transmitting devices. Wet clutches transmit torque from a first shaft to a second shaft by means of friction. Wet clutches are often used in power transmissions particularly of off-road vehicles, earth moving vehicles, bulldozers, tractors and agricultural machines as well as in stationary production machines and the like to selectively engage a device. In the engaged state a piston, which is movably received by a clutch chamber such as a cylinder, presses a first set of clutch elements, which are connected to the first shaft, towards a second set of clutch elements, which are connected to the second shaft, for transferring torque. In order to provide the necessary pressure, first a filling space between the piston and the clutch chamber is filled with suitable oil or other liquid, so that the piston is displaced by the oil from a disengaged state towards the clutch elements (filling phase). When the pressure inside the filling space is increased further, the piston is pressed against the clutch elements in a slip phase. The pressure increases until a third phase is reached in which the revolution speed of the first shaft (input shaft) corresponds to the speed of the second shaft (output shaft). When the pressure applied by the piston and the position of the piston is mainly constant and the revolution speed of the first shaft corresponds mainly to the second shaft, the filling and slip phases are over and the engaged state is reached. In the disengaged state the piston should be positioned as far as possible from the clutch elements, thus avoiding losses due to viscous friction of the hydraulic oil between the clutch elements or a slow rotation of the second shaft.
Brakes on wheels or moving parts of machines apply a negative torque, e.g. by pressure between brake shoes or linings and a part of the wheel or a moving part of a machine. Brakes are usually applied by pressurising a brake cylinder chamber and a piston applies force to the brake shoes or linings. Rapid braking can cause wheel lock up and skidding in an automotive vehicle or excessive stresses in a machine. Too slow an application of the brakes can result in dangerous situations developing.
In a clutch torque is transferred between a first shaft and a second second shaft, i.e. between a first torque transmitting element and a second torque transmitting element. In a brake torque is transferred between a first torque transmitting element, e.g. brake shoe or lining and a second torque transmitting element, e.g. a friction plate attached to a wheel.
In order to provide a fast coupling of the first torque transmitting element and a second torque transmitting element, e.g. between the first shaft to the second shaft, it is necessary to apply a high filling pressure to the piston during the filling phase leading to a fast displacement of the piston. But when the pressure is chosen too high, the piston makes suddenly brutal contact with the torque transmitting elements such as the clutch elements or brake elements resulting in undesired high peaks in the transmitted torque, e.g. possibly stalling of the engine, skidding of the vehicle or damage of a machine. In particular, there is not only a need to apply a clutch safely and quickly but there is also a need for emergency braking which is rapid but safe.
From US 2008/0076631 A1 it is known to determine a suitable filling pressure for the filling phase during a first calibration of the wet clutch by means of a trial and error method according to a Newton fix point iteration. The applying of the filling pressure is stopped automatically, when a predetermined threshold pressure between the piston and the clutch elements has been reached. The empirically determined filling pressure is stored in a control device for a valve connecting the filling space with a liquid pressure source, so that at every subsequent coupling during normal operation by an operator this filling pressure is used.
From U.S. Pat. No. 6,216,074 B1 it is further known to determine the coupling time between initiating the filling phase and reaching the engaged state. If the determined time is too long, the filling time, during which the filling pressure is applied to the piston, is lengthened.
It is a disadvantage of such kinds of methods for controlling a wet clutch that they cannot take into account changing environmental conditions like varying temperatures and system variations like wear of the friction plates in wet clutches. Therefore these methods are conservative in engagement time. There is a permanent need to shorten the coupling time without increasing the risk of an uncomfortable coupling behavior, like a brutal contact between the piston and the clutch elements, and therefore it is desirable to take into account the changing environmental conditions and system variations.
A further problem associated with prior systems is that the control of torque transferring devices such as brakes or clutches in large equipment is a safety critical application. This has lead to a very conservative approach with respect to the number of parameters that can be changed to provide an optimum adaption of the control signal for the filling phase to the prevailing conditions, e.g. wear of clutch plates or brake shoes. It would be desirable to have more degrees of freedom for adaption of the control signal while maintaining a secure application.